During VLSI fabrication, meticulously clean silicon wafers are critical for obtaining high yields and suitable performance characteristics of semiconductor devices. Removal of impurities from the wafer surface is important because impurities may diffuse into the semiconductor substrate during subsequent high-temperature processing, altering the substrate bulk and surface properties. Some impurities are donor or acceptor dopants which directly affect device performance characteristics. Other impurities cause surface or bulk defects such as traps, stacking faults or dislocations. Surface contaminants such as organic matter, oil or grease lead to poor film adhesion. The various types of impurities and contaminants must be removed by careful cleaning, such as chemical or ultrasonic cleaning at initiation of silicon processing and in various appropriate steps during processing.
Silicon processing typically begins with a cleaning step involving wafer scrubbing to remove loose particulate contaminants. Particulates are bits of material present on a wafer surface that have easily definable boundaries such as various dusts (atmospheric, silicon and quartz), link, photoresist chunks and bacteria. Particulates are generally removed using a process herein called a cleaning process. Material that is too small to be measurable is herein referenced as "material", which is generally removed using a polishing process.
Subsequent to a cleaning process, treatment with organic solvents, such as trichloroethylene, acetone, p-xylene, methanol and ethanol, is performed to remove organic impurities such as hydrocarbons and greases which remain from a prior wafer-grinding process. A final cleaning step includes treatment with several various inorganic chemicals to remove heavy metals, for example. These inorganic chemical mixtures are strong oxidants, which form a thin oxide layer at the wafer surface. This oxide layer is stripped, removing impurities absorbed into the oxide layer.
Chemical cleaning for removing chemically bonded films from wafer surfaces is one step in a cleaning process. Conventional chemical cleaning includes a series of acid and rinse baths.
Various silicon wafer cleaning systems are commercially available which clean wafers using mechanical scrubbing. A conventional silicon wafer cleaning machine utilizes a polishing pad affixed to a rotating turntable wherein the polishing surface of the polishing pad faces upward. The rotating turntable is commonly rotated at various controlled speeds, for example from 10 to 100 RPM, in a controlled clockwise or counterclockwise direction. A silicon wafer, generally in the form of a fiat, circular disc, is held within a carrier assembly with the substrate wafer face to be polished facing downward. The carrier assembly is affixed to an arm and lever so that a downward force is applied to the silicon wafer against the polishing pad. In some systems, the carrier assembly is motorized so that a rotational motion is applied to the silicon wafer. The wafer is also rotated by the carrier assembly at various controlled speeds in a controlled clockwise or counterclockwise direction. The relative speeds and rotation directions of both the turntable and the wafer are controlled independently so that the speeds and rotation directions may be the same or different.
The polishing pad and turntable are much larger than the silicon wafer. For example, a typical diameter of the pad and turntable is 22 inches while a wafer commonly has a diameter of approximately 10 inches. The carrier assembly is positioned in various places with respect to the pad and turntable using a mechanism such as a robotic arm. During a wafer polishing process, the carrier arm and wafer is moved about to various positions overlying the polishing pad.
The polishing process operates by rotating a polishing pad and bringing a silicon wafer into contact with the polishing pad as a liquid solvent slurry is applied. The silicon wafer contacts the polishing pad under pressure of a downward force applied to the silicon wafer. The amount of downward pressure applied to the carrier assembly is controlled. A mechanical cleaning process cleans by placing various solvents in the slurry into motion. One slurry typically includes a solution of silicon dioxide and potassium hydroxide. In another example, slurry is composed of silicon dioxide and ammonium hydroxide or some other amine. The moving solvent aids in removal of material. The combined action of the applied downward force, rotating actions of the wafer and the polishing pad and the physical-chemical action of the polishing slurry results in the removal of material from the substrate wafer.
The polishing pad is typically fabricated from a polyurethane and/or polyester-based material. The polishing process degrades the polishing material, reducing polishing performance. To restore the polishing material and improve polishing performance, a conventional polishing machine periodically reconditions or dresses the pad. The reconditioning process involves application of an abrasive material, such as a diamond surface including sharp particles or structures, to the pad. The abrasive material is used to erode the surface of the polishing pad in a controlled manner, thereby reviving the pad surface and restoring polishing performance. A conventional polishing apparatus uses a separate abrasive assembly held by an arm which extends approximately from the center of the turntable radially outward. The abrasive assembly is controlled to move back and forth, over the rotating turntable to restore the pad as the turntable spins and away from the turntable when restoration is complete.
After the wafer is polished, removed particles are washed from the wafer surface by transferring the wafer to a separate washing apparatus. Thus the wafer polishing process includes two steps, a polishing step and a washing step.
Performance of the polishing step depends on the relative motion of the polishing pad and the substrate. Particulate removal varies with the linear velocity of the wafer which moves with respect to the polishing pad. For any point on a rotating body, angular velocity can be converted to linear velocity. The linear velocity depends not only on the angular velocity but also on the distance of the point from the center of rotation. If the distance is doubled for the same angular velocity, the linear velocity of the point is doubled. One problem with conventional wafer cleaning systems is that the linear velocity at any point on the wafer can change rapidly, a local acceleration that stresses the wafer surface. A consequence of the high local accelerations on the wafer surface is that a greater amount of material is removed at a point, reducing polishing uniformity. Another problem that arises with conventional wafer cleaning systems is that the linear velocity of any point on the wafer, relative to the pad, cannot be suitably controlled. The independent rotational motion directions and angular velocities and the variable relative positioning of the wafer and pad engender a highly complex dynamic system which is difficult to model and control. One consequence of the dynamic complexity of the conventional polishing system is that the wafer is not polished uniformly. Polishing specifications typically require a high uniformity to tight tolerances. For example, silicon dioxide processing typically specifies the removal of approximately one micron of film. Often less than one micron of film is specified to be removed for metal applications. Another consequence is that a much longer polishing time is required, reducing semiconductor fabrication efficiency and productivity and increasing manufacturing costs.
An additional problem is that, just as the dynamic complexity of the conventional system makes uniform polishing of the wafer difficult, uniform degradation and restoration of the polishing pad are similarly rendered onerous. The combination of nonuniformity of polishing and nonuniformity of the condition of the polishing pad make the specification of high uniformity to tight tolerances very difficult to achieve.
Furthermore, the local accelerations that stress the wafer surface also cause local stresses on the polishing pad, tearing at the pad and reducing the operational life of the pad. High wear and tear on the polishing pad diminishes fabrication productivity due to down time of the polishing apparatus and increases manufacturing costs both because of the reduced operational time and the cost of replacing polishing pads.
Another problem is that conventional wafer cleaning systems utilize a polishing pad which is much larger than the substrate wafers. Slurry must be generously applied to the entire pad so that the large pad necessitates the usage of large amounts of chemical solvents, making the cleaning process sloppy, increasing cleaning costs and maintenance costs (due to the corrosive character of many solvents), and increasing the usage and therefore the cost of chemicals.
A further additional problem arises with respect to the slurry application in a conventional wafer cleaning system. The downward pressure of the carrier assembly and wafer on the rotating polishing pad is concentrated at the center point of the wafer. Therefore, slurry is forced away from the center of the wafer so that the slurry is not applied uniformly to the wafer.
Usage of a polishing pad in the form of a large thin disk makes replacement of the pad difficult. The polishing pad is affixed to the turntable with an adhesive. The pad is removed by merely tipping up the pad, thereby releasing bonding of the pad. Unfortunately, after polishing of numerous wafers, the pad is thoroughly soaked with slurry which is often toxic, volatile and corrosive.
In addition, utilization of a large polishing pad requires the cleaning system, as a whole, to be very large, requiring a large mount of floor space and thereby increasing manufacturing costs. Furthermore, complete enclosure of the large systems is difficult, making ventilation of toxic and unpleasant chemicals burdensome and expensive.